The present invention relates to a circuit for preventing a surge from occurring for use in an electromagnetic induction cooking apparatus, and more particularly to a surge preventing circuit for an electromagnetic induction cooking apparatus which detects an overheating of a heating plate or an upper plate disposed within the electromagnetic induction cooking apparatus or an overheating of a collector voltage of a power transistor and then cuts off its output slowly not abruptly, thereby preventing the breakdown of circuit components which may occur by virtue of a generation of a great surge.
A conventional control circuit for an electromagnetic induction cooking apparatus is shown in FIG. 1.
The conventional control apparatus comprises a low-pass filter 1 for passing only a low-band component of an alternate current (AC) power therethrough, a full-wave rectifying unit 2 for full-wave rectifying an output power of the low-pass filter 1, an inverter circuit unit 3, consisting of a working coil, resonance capacitors and a power transistor, for receiving an output power of the full-wave rectifying unit 2 as an operating power to heat a magnetic vessel, an input current sensing unit 4 for sensing an input current of the low-pass filter 1, a power supply 5 for producing direct current sources V.sub.CC1 and V.sub.CC2, and a direct current source V.sub.CC3 of an output control circuit by receiving the alternate current (AC), which are used to turn on or off the power transistors of the inverter circuit 3, a temperature sensing unit 6 for cutting off the input power source of the full-wave rectifying unit 2 by detecting an overheating of a heating plate which is resulted from the overheating of a full-wave rectification diode of the full-wave rectifying unit 2 and the power transistor of the inverter circuit unit 3, a microcomputer 7 for controlling the whole system and for outputting output control signals in response to the user's selection, an output converting unit 8 for converting the output control signal of the microcomputer 7 into an analog signal, an operational comparator OP.sub.1 for comparing the output signal of the output converting unit 8 with the output signal of the input current sensing unit 4, a vessel sensing unit 9 for determining whether a magnetic vessel, which is suitable to be heated, is laid on the upper plate by checking the output signal of the input current sensing unit 4, a condition sensing unit 10 for applying a vessel present/absent discrimination signal to the microcomputer 7 in response to the output signal of the vessel sensing unit 9, an integrator 11 for integrating the output signal of the operational comparator OP.sub.1, a negative feedback operating unit 12 for varying the output of the integrator 11 in response to the output of the vessel sensing unit 9, a charge/discharge unit 13 for charging the output voltage of the integrator 11, a search signal circuit 14 for converting the control signal outputted from the microcomputer 7 into a signal for driving the inverter circuit 3 and applying the converted signal to the charge/discharge unit 13 in order to discriminate the presence/absence of a vessel, an operational comparator OP.sub.2 for buffering and amplifying the output voltage of the charge/discharge unit 13 and outputting the amplified signal, a minimum turn-on time setting unit 15 for detecting the output voltage of the charge/discharge unit 13 when the output voltage is lower than a set value, a collector voltage limiting circuit 16 for detecting the collector voltage of the power transistor of the inverter circuit 3 when the collector voltage is greater than a rated value by the power transistor being turned on more than a predetermined time due to an abnormal operation, an upper plate temperature sensing unit 17 for detecting an overheating of the upper plate when the temperature of the upper plate rises over a set value, a NOR gate NOR.sub.1 for NORing the detection signals of the collector voltage limiting circuit 16 and the upper plate temperature sensing unit 17 and applying the NORed signals to an output terminal of the operational comparator OP.sub.2, a synchronizing circuit 18 for outputting a time point decision signal for turning on or off the power transistor of the inverter circuit 3, an operational comparator OP.sub.3 for comparing the output signal of the second operational comparator OP.sub.2 with the output signal of the synchronizing circuit 18, and a power transistor driving unit 19 for outputting a driving signal for turning on or off the power transistor of the inverter circuit 3 in response to the output signal of the third operational comparator OP.sub.3.
In such a conventional apparatus, when the temperature of the heating plate is abnormally high, the temperature sensing unit 6 detects such an abnormal condition and cuts off the input power of the full-wave rectifying unit 2 so that an excess inverse electromotive force is produced at the inverter circuit 3, thereby causing the power transistor to be broken down. In addition, there has been another disadvantage in that when the collector voltage of the power transistor of the inverter circuit 3 rises more than a set value, the collector voltage limiting circuit 16 detects the condition and outputs a high level detection signal, and when the temperature of the upper plate rises more than a set value, the upper plate temperature sensing unit 17 detects the condition and then outputs a high level detection signal, and also when the output voltage of the charge/discharge unit 13 becomes lower than a set value, the minimum turn-on time setting unit 15 detects the condition and then outputs a high level detection signal, and thus when any one of the minimum turn-on time setting unit 15, the collector voltage limiting circuit 16 and the upper plate temperature sensing unit 17 outputs a high level detection signal, the NOR gate NOR.sub.1 outputs a low level signal, causing the output signal of the second operational comparator OP.sub.2 to be a low level signal and thus a low level signal is outputted from the third operational comparator OP.sub.3, thereby the transistor driving unit 19 stops instantaneously the driving of the inverter circuit 3 so that a surge voltage is produced at the inverter circuit 3 which may cause the breakdown of the circuit components such as the power transistor and the like.